#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <algorithm>
using namespace std;

struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution
{
public:
    TreeNode *invertTree(TreeNode *root)
    {
        // 1.递归法
        // if(root == nullptr) return root;

        // swap(root->left, root->right);
        // invertTree(root->left);
        // invertTree(root->right);
        // return root;

        // 2. 层序遍历 + 交换左右子节点
        queue<TreeNode *> que;
        if (root)
            que.push(root);
        while (!que.empty())
        {
            int size = que.size();
            while (size--)
            {
                TreeNode *node = que.front();
                que.pop();

                swap(node->left, node->right);
                if (node->left)
                    que.push(node->left);
                if (node->right)
                    que.push(node->right);
            }
        }
        return root;
    }
};